1. Field of the Invention
This invention relates to a high-density optical storage unit for storing and retrieving electronic information, including encoded data, text, image, and audio information, and a method for writing and reading the information. "High-density" in this context shall mean densities of stored bits of information in excess of 10.sup.9 bit/cm.sup.2.
2. Description of the Prior Art
In conventional optical storage units, the shape and the size of the stored bits are defined by the narrow focal point of a laser beam, making the circular bit regions about 1 micrometer in diameter. This means that the storage density is limited to about 10.sup.10 bits/cm.sup.2. The well-known (read-only) compact disk (CD) can store approximately 10.sup.10 bits of information on its entire active surface, for example.
Regarding the storage media used in conventional erasable (read/write) storage units, there are essentially two groups of leading optical contenders each requiring its own technique for reading and writing information: Magneto-optic and phase-change materials. Both techniques employ glass or plastic disks coated with thin films of storage material; they depend on lasers for recording, yet their approach to writing and reading information is markedly different.
As is well known (e.g., from J. C. Iwata, "Optical Storage," IBM Research Magazine, Vol. 25, No. 1, pp. 4-7, 1987), magneto-optic recording relies on heating, by a laser beam, and in the presence of an external magnetic field, a thin film of magnetic material coated onto a substrate. As the temperature of the film is locally raised above the Curie point of the material, the external magnetic field will reverse the original direction of the magnetization at the particular location, and as the spot involved cools, the new direction of the magnetization is "frozen," thus storing a bit of information.
The stored information is read by flashing a laser beam, though at reduced power, onto the storage medium causing those storage locations holding magnetization with a changed direction to slightly rotate the plane of polarization of the reflected beam, a phenomenon known as the Kerr effect. This rotation can be sensed by a photodetector and the stored bit identified.
Erasure of the stored information is done by simply heating the particular storage area to a temperature above the Curie point in the presence of a magnetic field having the original direction.
In phase-change recording, a short (less than 100 ns) burst of laser light converts a tiny spot on the media's highly reflective crystalline surface to the less reflective amorphous, or semicrystalline state, the conversion occurring upon rapidly heating the material to a temperature above its melting point, then rapidly quenching it, "freezing" it into the amorphous state.
For reading the stored information, a laser beam is scanned over the amorphous and crystalline storage locations; the variations of the reflected light are detected and the locations storing a bit of information identified.
Restoring the storage medium to its original state is done by heating the bit locations to a temperature below the material's melting point, but for an "extended" period of time (on the order of 10-.sup.5 S).
Both these techniques have the severe disadvantage of being limited in miniaturization by diffraction to a bit size of about .lambda./2.
Under the present invention, several recording schemes are conceivable, and two such schemes and the appertaining storage media will be discussed below by way of example. The first scheme to be discussed operates with a thermoplastically deformable storage material in which the bits of information are stored in the form of tiny dints produced by heat and pressure. The second scheme is an electro-optical system using a storage material which has the capability of trapping electrical charges when illuminated by light having a sufficiently short wavelength.
The feature common to these schemes is the accessing in two discrete steps: In a first step, light selects an area of a few square micrometers as determined by the diffraction limit, and in a second step, a small protrusion selects a bit of much smaller size, say as small as a fraction of 0.01 .mu.m square, within said area. In this manner, a very large number of tips, potentially millions, can be operated in parallel.
Work on surface modification by means of a laser-heated tip pressed into a thermoplastically deformable material was reported by H. J. Mamin and D. Rugar in their abstract "Laser-Assisted Nanolithography with an AFM," Bull. Am. Phys. Soc., Vol. 37 (1992) p. 565/6, paper No. M28 5.
Writing information into storage by means of charge injection with a single tip and silicon nitride as the storage medium is known from R. C. Barrett and C. F. Quate, "Charge storage in a nitride-oxide-silicon medium by scanning capacitance microscopy," J. Appl. Phys. 70 (5), Sep. 1, 1991, pp. 2725-2733.